CT Simulator Visit
After the mask is made, the next step is usually planning your treatment on the CT Simulator. This is an x-ray scan taken with you lying on a couch in the position necessary for your treatment. Your radiographers will explain everything in detail at the start.
â€¢ You will wear your mask during the scan to keep you in the correct position. Your radiographers will fit your mask and then attach a â€˜Localiserâ€™ box over it. It is opaque and you will not be able to see through it. This box will cover the whole of your head although it wonâ€™t touch you. The planning computer uses this localiser box to calculate where your tumour is.
â€¢ If you were fitted with a mouth bite you will have it in place during your scan. You will be able to move your lower jaw, so you will still be able to swallow.
â€¢ For this scan, most people will need an injection of contrast (dye) to help show up the tumour more clearly. This will be done just before your radiographers finalise your planning position.
Your doctor or radiographer will put a small needle (cannula) into a vein, usually on the back of your hand. It will be taped in place and will stay in during the scan. We will then inject the contrast (dye) through this needle. It may feel a little cold but you should not feel any discomfort. The injection itself may be given by a machine whichis simply done for convenience.
Some people can have an allergic reaction to the contrast. This is rare but we will ask some questions before beginning to ensure it is unlikely to happen to you. Your doctor and radiographers will keep a close check on you during the injection and scan, so if you feel anything unusual please tell us. Your radiographers will then finalise your treatment position and leave the room to begin the scan. Scanning usually takes around five minutes and during this time the radiographers will watch you on closed circuit TV.
Once the scan is complete the cannula will be removed.
The data from your CT scan is then transferred to treatment planning computers where the tumour is delineated along with the healthy organs which need to be avoided.
Figure 1 shows one â€˜sliceâ€™ of the Radiotherapy CT scan. Volumes have been drawn onto the scan by the Clinical Oncologist. Firstly, the Clinical Oncologist draws the tumour volume, and then they will add a Margin around the tumour to create a Planning Target Volume (PTV), which allows for any potential random set-up errors that could occur. Since patients are immobilised in a shell and unlikely to move by much, the margin added to the tumour to create the Planning Target Volume (PTV) for brain radiotherapy is small.
Organs which will be avoided are also outlined.
Figure 1: patientâ€™s CT Scan with volumes drawn by Clinical Oncologist
The CT scan data can be viewed in a number of ways. Figure 1 show a Transverse slice through a patientâ€™s head. The data can also be viewed in a 3-dimensional fashion as shown in Figure 2, which shows rendering of the skull bones. Figure 3 is at a different angle and has the bones view turned off so that we can see the organs drawn on the CT scan from a 3-dimensional perspective.
Figure 2: 3-dimensional bone rendering of CT scan
Figure 3: 3-dimensional view of organs drawn by Clinical Oncologist
Once the volumes have been completed the next stage is the creation of a treatment plan. This is where the radiotherapy is modelled on a computer. The process allows the computer operator, a highly skilled professional planner, to find the optimum settings for the radiation beams to be used in the patientâ€™s treatment. The planner decides on the best size, shape, energy and arrangement of the beams then lets the computer calculate and display the dose.
Figure 4 shows a treatment plan. The blue areas are low radiation dose, green are higher dose and red is the full dose. The aim of the planning process is to focus the high-dose (red) areas around the Planning Target Volume (PTV) whilst keeping the dose as low as possible to healthy organs.
Figure 4: radiotherapy treatment plan with high dose focussed around PTV
In order to shape the beams to fit the Planning Target Volume (PTV) the planner makes use of metal leaves in the head of the treatment machine that can move in and out of the radiation beam. The leaves can be positioned to shield healthy tissue and so conform the radiation to the shape of the Planning Target Volume (PTV). Figure 5 shows a â€˜Beamâ€™s Eye Viewâ€™ of a radiation beam with the leaves fitted around the area to be treated.
Figure 5: Beamâ€™s Eye View
Once the radiation has been successfully modelled it is approved by the consultant Clinical Oncologist. The treatment plan is then sent electronically to the treatment machines. All of the parameters of the treatment plan, including the sizes of the beams, their angles and the shape of their leaves are loaded into the treatment machine so it reproduces the them exactly.
Checking is vital, so at every stage the data is independently verified. The radiation distribution created by the computer is checked by medical physicists and the machines are constantly monitored by electronic engineers.
In order to make the treatment as safe and effective as possible, once the patient begins their treatment their set-up is verified by taking electronic images of the radiation beamâ€™s eye view. At every stage the radiotherapy staff work to accuracy levels of millimetres.
Whether you will be receiving radiotherapy as an outpatient or an inpatient will depend on how fit you are and your ability to travel. Your doctor will discuss this with you and make specific arrangements. You will be given a fixed appointment time for your treatment and every effort is made to keep closely to these times. However inevitably, occasionally, there will be some delays.
While some people find the shell and treatment machines intimidating at first, you cannot feel the radiotherapy and there is no discomfort during treatment. You are not radioactive and there is no need to take any special precautions for the safety of others. You are not a hazard.
The side effects depend on how much of your brain is being treated or if the spinal canal has to be treated as well. Most side effects are quite mild and all efforts are made to minimise them. However, some are inevitable.
Once all the computer planning has been completed, you might return for a run through on the simulator but, simulators are gradually being phased out of radiotherapy because the â€˜simulationâ€™, which requires the taking of X-ray pictures and a run through of the set-up, can be done on the newer style treatment units so , you may just go straight onto the treatment units.
If you will be visiting the simulator machine below is a short description of what this involves:
A simulator machine is a special x-ray machine that can take films and reproduce the movements of the treatment machine and therefore 'simulate' the position of the x-ray beams that will be used for your treatment. You will need to lie in the treatment position wearing your newly made shell. The radiotherapist with the help of a radiographer plan the position of the radiotherapy beams using information from scans, operation and previous examinations. This session lasts about 30 minutes. It is often much longer than the treatment time on the machine. At the end of this simulator session you will be given a date and time for starting treatment. This may be a few days later.
This article has been written by Russell Fitchett, Superintendent Radiographer, Norfolk and Norwich University Hospital.
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